Utilizing Three-Terminal IBC-Based Si Solar Cells as a Platform to Study The Durability of Photoelectrodes for Solar Fuel Production

Unassisted, photoelectrochemical (PEC) reactions, such as H₂generation and CO₂reduction, are limited by the durability of the immersed semiconductor and catalyst. While great strides have been made to improve stability under illumination, dark stability remains relatively unexamined and presents great challenges. Typically, semiconductors require the use of protection layers to drive the PEC reactions of interest. Cathodic protection is an established electrochemical method for preventing electrode degradation in harsh conditions. Similar protection strategies cannot be applied to traditional two-terminal (2T) semiconductor photoelectrodes because of their inability to pass reverse bias current in the dark. New, three-terminal (3T) photovoltaic (PV) architectures can remove the limitations of traditional 2T photoelectrodes by the addition of an extra electrical contact which provides an alternative low resistance path to drive electrochemical reactions, even in the dark. 3T Si interdigitated back contacted (IBC) cells have n+ and p+ doped back contacts, similar to a traditional IBC cell, with an additional conductive front n+ contact to create the third terminal. Here, we investigate how 3T Si PV can be operated as photocathodes to understand new protection methods for photocathodes in aqueous environments without protection layers. We characterize performance of the 3T electrode in a regenerative redox electrolyte, methyl viologen, to decouple catalysis from photocathode device performance. The 3T Si electrode is used as a platform to understand the applications of cathodic protection to electrodes in photoelectrochemical systems. The 3T architecture provides additional capabilities to PEC systems such as cathodic protection, the ability to drive reactions with or without illumination, and in-situ switching between different operational modes. During experiments, the 3T photoelectrode is wired in the diode mode during illumination and wired in ohmic mode in the dark, where the cathodic protection is applied. We show that bare 3T-based Si photocathodes maintain PEC activity in methyl viologen electrolyte after several hours of light/dark cycling. This work helps advance PEC use in real-world conditions where durability under variable illumination must be considered. Future work includes modeling 3T architectures and cathodic protection in fuel forming reactions and experimentally adding Pt and Cu catalysts to extend this work to systems for H₂generation and CO₂reduction.